Generally, discrete transformers and optocouplers are used to transmit signals from one electrical circuit to another electrical circuit without direct electrical contact between the two electrical circuits. However, discrete transformers and optocouplers are relatively large, heavy, and expensive, and in the case of optocouplers, aging may be a problem.
Another way to transmit signals from one electrical circuit to another electrical circuit without direct electrical contact is a coreless transformer. While a discrete transformer comprises a core to direct the magnetic flux, the coils of a coreless transformer can be placed close enough to each other so that the core can be spared.